Superabsorbent polymers (SAP's) are materials that imbibe or absorb at least 10 times their own weight in aqueous fluid and that retain the imbibed or absorbed aqueous fluid under moderate pressure. The imbibed or absorbed aqueous fluid is taken into the molecular structure of the SAP rather than being contained in pores from which the fluid could be eliminated by squeezing. Some specialty SAP's can absorb up to 1,000 times their weight in aqueous fluid. The present application is directed to: (1) a graft polymerization method suitable for preparing biodegradable SAP's and biodegradable polymers suitable for other applications, and (2) the novel graft copolymers produced by these methods.
One method of producing a SAP involves graft polymerizing acrylonitrile onto a starch in the presence of an initiator, such as a ceric (+4) salt, to form a starch graft copolymer, and saponifying the nitrile groups with an alkali metal to form a saponificate having alkali carboxylate and carboxamide groups.
Saponification, however, requires expensive machinery and generates ammonia, which can be corrosive, costly to remove, and expensive to recover and/or dispose of. Also, potassium hydroxide (KOH) added during saponification makes the saponified starch graft copolymer mixture basic and provides a product mixture that is both viscous and sticky. An acid, e.g., hydrochloric acid, nitric acid, sulfuric acid, or phosphoric acid, must be added to the basic mixture to neutralize the excess base and adjust the mixture's pH to about 7.5 for most applications. Finally, the sticky and viscous material must be pumped into large volumes of methanol and undergo several chopping steps to remove dissolved salts and transform the polymer into a workable form. The resulting waste solutions can also be expensive to dispose of because they include potassium and ammonium salts and other extraneous materials. More particularly, wastes containing acrylonitrile can be hazardous and similarly expensive to dispose of. The isolation of one pound of polymer can require as much as 3 gallons of methanol. As a result, a 10 million pound SAP/year plant can require as much as 30 million gallons/year of methanol. The loss of only 1% of the required methanol could introduce as much as 300,000 pounds/year of methanol into the environment.
A more recent method described in U.S. Pat. No. 7,459,501 was developed for producing a SAP and involves graft polymerizing a monomer (acrylic acid or its ester, optionally including acrylamide) onto a starch in the presence of a cross-linker and an initiator under isothermal conditions. The batch method involved combining the reactants in water, heating the mixture to about 170° F., and maintaining that temperature for about 15 minutes. The resulting viscous mass was neutralized with base and isolated after adding large volumes of methanol to convert the viscous mass into a physical form that can be processed. Although this method of producing a SAP avoids a saponification process, handling acrylonitrile, and recovering large volumes of ammonia, the need to utilize and recover large amounts of methanol (or other lower alcohol) remains and the process cannot be carried out as a continuous process because of the viscous and sticky nature of the initially formed graft copolymer product. Without the use of an alcohol in the product isolation step, the polymer produced is not a flowable product and remains too viscous and sticky to be processed by currently available equipment, even in a batch process.
What is needed is a high purity biodegradable SAP and related polymers having a range of absorbances and other properties, and a method for making the polymers with high conversions from readily available starting materials. The process should be capable of being carried out in a continuous manner utilizing currently available production equipment to directly produce large volumes of the polymer and allow for its direct isolation without the need to handle toxic materials or recycle large volumes of hazardous gases and/or solvents. The present disclosure addresses these needs.